In response to a recent movement towards downsizing of electronics devices and automation of assembly, QFP (Quad Flat Package) type and BGA (Ball Grid Array) type semiconductor apparatuses in the shape of CSP (Chip Size Package/Chip Scale Package) are widely used. As semiconductor devices included in the semiconductor apparatuses perform high-speed and highly-functional signal processing, more external terminals become necessary. In view of this circumstance, especially a BGA type semiconductor apparatus, in which external connection terminals are disposed two-dimensionally along a bottom surface of the semiconductor apparatus, is popularly employed. Such BGA type semiconductor apparatus is taught in Patent Document 1, Japanese Unexamined Patent Publication No. 121002/1997 (Tokukaihei 9-121002) (published on May 6, 1997), for example. In the semiconductor apparatus of Patent Document 1, a semiconductor chip, with its surface on which a circuit is formed facing upward, and the wiring board are wired by using a wire bonding method, thereby allowing the semiconductor chip to be electrically connected to external connection terminals via a wiring pattern.
FIG. 5 shows a cross section of an exemplary structure of a resin-sealing type semiconductor apparatus in the shape of BGA (BGA type semiconductor package, BGA type semiconductor apparatus). This is currently a majority of BGA type semiconductor packages. Compared with other semiconductor apparatuses, the BGA type semiconductor apparatus has an advantageous structure in view of electric properties and downsizing of semiconductor apparatuses. For this reason, the BGA type semiconductor apparatus is popularly employed in portable electronics devices such as portable phones and portable game machines.
The following describes a BGA type semiconductor apparatus 100, with reference to FIG. 5. As shown in FIG. 5, the BGA type semiconductor apparatus 100 includes, mainly, a semiconductor chip (semiconductor device) 1, a wiring board 80, a thin metal wire (wire) 7 for connecting the semiconductor chip 1 and the wiring board 80, and an external terminal 9 having conductivity. The semiconductor apparatus 100 is sealed with a resin 10. The wiring board 80 includes an insulative core substrate. A wiring pattern made of copper foil is formed on both surfaces of the core substrate. The wiring pattern on the upper surface of the wiring board 80 and the wiring pattern on the lower surface of the wiring board 80 are electrically connected via a through hole. The through hole is formed by making a vent with the use of a drill or the like and plating an interior of the vent with copper. Further, the wiring board 80 is covered by insulative solder resist, except for a terminal section for wire bonding in the wiring pattern, which terminal section is provided in the form of a circuit, and a land section in the wiring pattern, on which land section external terminals are to be formed.
As electronics devices are becoming smaller and thinner in recent years, semiconductor apparatuses to be installed in the electronics devices have been required to become smaller and thinner. This gives rise to an increasing demand for a thinner wiring board 80, which is to be used in the semiconductor apparatus 100. Specifically, an insulative core substrate with the thickness of 200 μm or below has become a majority. An insulative core substrate with the thickness of approximately 40 μm to 60 μm is also employed popularly. In the case of a wiring board including the core substrate with the thickness of approximately 40 μm to 60 μm, the thickness of the wiring board, including solder resist, is approximately 100 μm.
In the BGA type semiconductor apparatus 100, the semiconductor chip 1 is mounted on a surface of the wiring board 80 by use of an adhesive, which surface is opposite to a surface on which a circuit is formed. A pad section of the semiconductor chip 1 and a wire-bonding terminal section of the wiring board 80 are connected via the thin metal wire 7, which is conductive. As electronics devices are becoming highly functional in recent years, there arises a case in which a plurality of semiconductor chips 1 are placed one above the other. The thickness of the semiconductor chip 1 is generally in the range of 70 μm to 400 μm, but the thickness depends on the number of installed semiconductor chip 1. To adhere the semiconductor chip 1, an adhesive such as silver paste, insulation paste, and sheet adhesive is used. In recent years, sheet adhesives are popularly used in order to improve adhesiveness between the semiconductor chip 1 and the wiring board 80. A sheet adhesive is supplied either by adhering it in advance either to the semiconductor-chip mount area of the wiring board 80 or to a rear surface of the semiconductor chip 1. One way of applying the sheet adhesive to the rear surface of the semiconductor chip 1 is that the sheet adhesive is adhered to a rear surface of a wafer, and then the wafer with the sheet adhesive are diced into chips. Another way of applying the sheet adhesive is to transfer an adhesive component of a dicing sheet to the rear surface of the semiconductor chip 1.
The thin metal wire 7 for electrically connecting the semiconductor chip 1 and the wiring board 80 is made of a material such as gold and copper. Specifically, a gold wire with a cross-sectional diameter of approximately 20 μm to 30 μm is popularly employed currently.
In the semiconductor apparatus 100, the semiconductor chip 1 and the thin metal wire 7 are sealed with the resin 10 in such a way as to be covered by the resin 10. The sealing is performed by, for example, a transfer molding method using resin. A thermally-curable epoxy or biphenyl resin is popularly used as the resin 10 for sealing.
Furthermore, in the semiconductor apparatus 100, the external terminal 9, which is made of a metal, such as a solder ball is bonded to an opposite surface of the wiring board 80 by reflowing. The diameter of the solder ball differs depending on a pitch of the external terminal, for example. A material of solder of such solder ball has been shifted recently from a eutectic solder to a lead-free solder in view of environment protection. The lead-free solder is higher in melting point than the eutectic solder. Thus, bonding the lead-free solder requires a higher temperature than bonding the eutectic solder. Further, there is also a solder-ball terminal having a metal ball, such as copper, or a resin ball, such as resin, at the center thereof, in order to keep a clearance between the semiconductor apparatus and a mount board when the semiconductor apparatus is mounted on the mount board.
The foregoing describes a structure of the BGA type semiconductor apparatus. There is also a semiconductor apparatus called CSP, relatively similar in dimension to a semiconductor chip, that has the above structure. There is also a semiconductor apparatus having an external terminal formed without using a metal ball such as solder. Instead, a solder paste or the like is applied and then melted so as to form an external terminal of 0.1 mm or below. Another semiconductor apparatus is an LGA (Land Grid Array) type semiconductor apparatus in which no solder is provided to a metal land of a board.
To mount such semiconductor apparatuses on a mount board, the following method is commonly used. First, a solder paste or a flux is applied to the mount board. Then, a semiconductor apparatus (package) is placed thereon. Thereafter, an external terminal made of a solder is melted by use of a heating machine such as a reflow oven, thereby connecting the semiconductor apparatus to the mount board. As described above, a solder material that is a component of the external terminal has been shifted in recent years from the eutectic solder to the lead-free solder in view of environment protection. For this reason, the temperature of the heat to be applied in mounting the semiconductor apparatus on the mount board tends to increase. Specifically, change in the solder material from the eutectic solder to the lead-free solder causes a reflowing temperature in mounting the semiconductor apparatus on the board to increase by approximately 20 degrees to 30 degrees.
As described above, to form the external terminal in producing the BGA type semiconductor apparatus, the following method is popularly employed. Specifically, the solder to be used as the external terminal is melted by use of a heating oven such as a reflow oven. Further, as also described above, to mount the semiconductor apparatus in the shape of a package such as BGA and LGA on the mount board, it is common to melt the solder and other components together in a heating oven such as a reflow oven. As the solder has been shifted in recent years from the eutectic solder to the lead-free solder in view of environment protection, the temperature of the heat to be applied in the heating oven tends to increase.
Accordingly, the semiconductor apparatus is required to be reliable enough not to cause a defect due to applied heat. A concrete defect due to applied heat is shown in the lower diagram of FIG. 5. Specifically, applied heat causes expansion of moisture absorbed by the semiconductor apparatus (semiconductor package). As a result, the inside of the package expands, causing the defect. In a semiconductor apparatus in which a semiconductor chip is placed on a wiring board and sealed with resin, moisture that is absorbed after assembly is completed tends to accumulate, with the center at an area where the semiconductor chip and the wiring board are bonded together, in the vicinity of interfaces of the components. The moisture tends to accumulate especially in an interface area of the semiconductor chip and the wiring board. For this reason, a most frequently-caused defect is expansion of a part, under a semiconductor-chip mount section, of the wiring board due to heat of a reflow oven, which heat is applied in order to mount the semiconductor apparatus (semiconductor package) on the mount board or in order to mount the solder ball. This occurs more in semiconductor apparatuses using a thin wiring board. If the wiring pattern includes a closed area under the semiconductor-chip mount section, the expansion of the board that occurs in the above situation is likely to originate in the closed portion.
As the foregoing describes, expansion of moisture having been absorbed inside causes defects, such as change in outer shape of the semiconductor apparatus, which change causes the semiconductor apparatus to become a defective or to become no longer attachable, and disconnection of a wiring inside. It is thus demanded that no such defect is produced in the semiconductor apparatus.
This tendency of expansion in the semiconductor-chip mount area of the wiring board increases especially in the semiconductor apparatuses (semiconductor packages) having a thin wiring board. This is an obstacle to making the semiconductor apparatuses thinner and smaller.
To solve this problem, for example Patent Document 2, Japanese Unexamined Patent Publication No. 15628/2001 (published on Jan. 19, 2001) teaches a semiconductor apparatus 101 shown in FIG. 6. In the semiconductor apparatus 101, a vent (vent hole) 11 for releasing the air is made through a part, within the semiconductor-chip mount area 20, of the wiring board so as to allow moisture accumulated inside the package to escape.
However, the semiconductor apparatus 101 having the vent 11 for allowing moisture accumulated inside the package to escape requires a space in the wiring board 81 for the vent 11 to be provided. This gives a restriction in layout of a wiring that is necessary to function as a device. To enable mass production of the semiconductor apparatus 101 under the current situation, a margin is necessary between the vent 11 and a neighboring wiring. Concretely, if the diameter of the vent is approximately 0.1 mm, the margin of {accuracy in position of the vent}+{distance to prevent solder resist from covering the vent}+{distance to assure the solder resist to cover the neighboring wiring} is necessary. In this case, there exists an area, with the diameter of approximately 0.3 mm, where the wiring cannot be formed. If the margin is tightly set, production cost of the wiring board 81 increases. On the other hand, if the margin is loosely set, the area where the wiring cannot be formed becomes wider.
Furthermore, a step of making the vent 11 needs to be added to the production process of the wiring board 81. This also causes an increase in production cost of the wiring board 81.
There may be a case in which metal in the semiconductor-chip mount area is arranged in solid entirely. There may be another case in which no metal pattern is formed. In these cases, however, the degree of adhesion between the semiconductor chip and the wiring board is too strong. This causes moisture in the package to accumulate in an area other than the semiconductor-chip mount area. Consequently, expansion occurs, originating in the area where the moisture has accumulated.
Meanwhile, there is another way to improve the degree of adhesion between a semiconductor chip and a wiring board in a semiconductor apparatus in which the semiconductor chip is bonded to the wiring board with the use of a sheet adhesive. Specifically, solder resist is applied for plural times so as to reduce protrusions and depressions that appear due to the wiring board, thereby lessening the influence of the protrusions and depressions of a wiring pattern thereunder. In this method, however, the step of applying solder resist is performed for plural times in producing the wiring board. This increases the number of steps in producing the wiring board, and therefore causes an increase in costs.
Further, there has been suggested another method in which, prior to assembly of a semiconductor package, accumulated moisture is removed in advance from the wiring board by heating or reducing pressure. This, however, also causes production cost or assembly cost of the wiring board to increase.